It is well-known to convert multiple-bit image data into 1-bit image data comprising “on” and “off” pixel values to enable an image having varying intensity levels to be printed so that the printed image is constituted by a plurality of dots, the number density or size of which varies in order to represent the varying intensity levels.
Such programs fall into two broad categories. The first category carries out so-called “amplitude modulated” (AM) screening to generate 1-bit image data that, when printed, produce an image that is constituted by a plurality of sizes of dots regularly arranged throughout the image. The second category of programs carries out so-called “frequency modulated” (FM) screening to generate 1-bit image data that, when printed, produce an image that is constituted by a plurality of number densities of dots, substantially all of the dots typically having the same size.
FM screening is widely considered to provide the potential for higher quality image reproduction than AM screening, but has not been widely adopted. One of the issues affecting FM (also known as stochastic) screening has been the graininess (noise and patterning) of the printed result. Various proposals have been made to reduce or eliminate noise in FM screened images, but these techniques have only been developed in relation to monochrome printing. The techniques do not eliminate noise that can arise in FM colour images or colour errors which can occur (especially at mid intensity levels) as a result of mis-registration of the arrays of different coloured dots laid down in the printing process.